belief is an arbitrary decision to insist something is true
I think that's a little too narrow a definition; the word for what you're talking about is "faith." I've often been asked if I "believe in evolution," and although the choice of words makes me cringe, the short answer has to be "yes." The longer answer is: I believe in evolution (or gravity, for that matter) the same way I believe in Philadelphia. Now, I don't know that Philadelphia exists. I've never been to Philadelphia. I've heard about it, and read about it, and seen road signs in pointing toward it, and even known a number of people who claim to have lived in it, but in answer to Ken Ham's famous question, no, I wasn't there. I have no personal proof that it exists, and yet I believe there is a city called Philadelphia. And I will insist pretty strongly that this belief is true, but there's nothing arbitrary about it.
There are alternate explanations, of course. Perhaps Philadelphia did exist up until five minutes ago, but no longer does. Perhaps there was never a Philadelphia, but someone decided there was money to be made by pretending there was, and put together an elaborate deception to convince people of it. Perhaps it's all just a mass hallucination. But the simplest and most rational interpretation of the evidence is that Philadelphia exists ... which is the foundation of my belief.
So what I wrote in my previous post, "the same way you believe in gravity," may not have been quite right. What I should have said was probably something like "just as strongly as you believe in gravity," because the beliefs stem from different sources, one from faith and one from evidence. But "God created the world in six days, six thousand years ago" and "gravity exists" are both statements of belief. And if you don't understand that both beliefs are held with equal sincerity by large numbers of people, you will consistently underestimate those who hold to the former.